1. Field of the Invention
This invention relates to a composition and method for completing offshore wells. More particularly, the invention relates to such a composition and to a method for use of the composition in the cementing of casing of such wells wherein there is injected into the annular space between the sidewall of the well and the casing, after completion of the drilling operation and placement of the casing in the well and just prior to placement of the cement slurry therein, a marker fluid which is distinctively different in color from both the cement slurry and the drilling fluid and therefor visually distinguishable therefrom.
2. Description of the Prior Art
Drilling an offshore well presents many difficulties not encountered in drilling wells onshore. In drilling an offshore well, drilling is usually started by lowering to the bottom of the body of water, from a platform or a floating vessel, a drill string having attached to the bottom thereof a pilot drill bit to drill a pilot hole 20 to 200 feet deep. This pilot hole is then enlarged to a diameter of about 36 inches by successively redrilling the hole with one or two larger diameter bits. Salt water is used as a drilling fluid during drilling of this pilot hole. The salt water is circulated down the drill string, through the drill bit, up the annulus between the sidewall of the borehole and the drill string and out into the surrounding water. The salt water in the bore hole is then displaced by drilling mud. A conductor pipe is lowered into the borehole and cemented in place by displacing the drilling mud by circulating a cement slurry down the casing itself or down the drill string, out a guide shoe on the bottom of the drill string, up the annulus and out into the surrounding water. When the annulus has been filled with cement slurry, the well is shut in until the cement sets. Drilling is then continued to a depth of about 2,000 to 5,000 feet and the next string of casing referred to as "surface casing" cemented in place much as described above. To complete a well it is necessary to subsequently drill to greater depths and cement in place a string of intermediate casing and finally a liner.
In the above-described cementing operations, one of the more difficult aspects is in determining when all of the drilling fluid has been displaced from the annulus by the cement slurry, i.e., when the annulus is filled with cement. The fluids are displaced from the annulus out into the surrounding water at the top of the borehole. Both the drilling fluid and the cement slurry are opaque fluids with approximately the same color. Thus, the water around the borehole in which casing is being cemented becomes quite murky. This makes its difficult for either divers or underwater cameras to detect when all of the drilling fluid has been displaced and cement slurry has filled the annulus and is starting to emerge from the annulus. For a competent cement job, it is essential that the cement slurry completely fill the annulus. At the same time it is not desired to waste cement slurry or rig time by pumping an excessive amount of valuable material out into the surrounding water.
Additionally, in the upper portions of a well the integrity of the reservoir being drilled can be quite low and unpredictable. The cross-sectional area of the annulus may vary considerably at different depths and be difficult to measure. Therefor, it is difficult to calculate the volume of the annulus which would indicate the volume of cement slurry required.
Broadly, hydraulic cement is made by blending together a calcareous material such as limestone, and an argillaceous material, such as clay or shale and feeding the blend into a rotary kiln at high temperature where it fuses into a material called cement clinker. The clinker is cooled, pulverized and blended with a small amount of gypsum which controls the setting time of the finished cement. When mixed with water, the clinkered products hydrate and form a hardened cement. Over the years a very large number of materials have been added to cement slurries in an attempt to modify their properties and/or setting characteristics. Such additives include setting time accelerators, setting time retarders, light weight additives, heavy weight additives, lost circulation control agents, filtration control agents, friction reducers and the like. In some cements not used in wells, coloring agents have been used to impart an attractive appearance to the set cement.
U.S. Pat. No. 3,376,146 to Mitchell covers a low density cement composition for use in wells comprising an aqueous slurry of Portland cement and particulate carbon, e.g., ground anthracite coal, lignite coal, bituminous coal, graphite, petroleum coke or coke derived from coal.
U.S. Pat. No. 2,609,882 to Morgan et al. describes a composition for cementing wells which composition overcomes the adverse affect of lengthening the setting time of cement slurry contaminated with drilling fluid containing certain additives such as caustic or quebracho. The cement composition comprises an aqueous slurry of Portland cement and activated carbon, such as that derived from coconuts, cane sugar, kelp, coal, lignite and the like.
U.S. Pat. No. 4,006,031 to Ferch et al. describes an aqueous dispersion of carbon black and a fluoride-containing anionic or nonionic surfactant for adding to a mineral binding system such as clear concrete to impart outside weather stability and durable black pigmentation.
U.S. Pat. No. 3,117,882 to Herschler et al. covers a colored cement composition containing a coloring composition comprising a substantially water-insoluble pigment such as carbon black admixed with a dialkyl sulfoxide.
U.S. Pat. No. 4,043,828 to Gaines covers a cement slurry having a reduced tendency to shrink prior to hardening. The slurry contains, in addition to a hydraulic cement and water, a particulate carbonaceous material such as delayed coke or naturally occurring coal, for example, anthracite, bituminous or lignite coal.
U.S. Pat. Nos. 4,069,870, 4,114,692, 4,144,077 and 4,200,153 to Gallus concern a high temperature cement composition for wells containing, as an additive to resist degradation of the compressive strength, carbon black, anthracite, calcined coke, uncalcined coke, burned oil shale or raw oil shale.
While each of the aforementioned compositions has met with some success in particular applications, the need exists for a further improved composition useful not as a cementing composition itself but in placing a cementing composition in a desired location.
Accordingly, it is a principal object of this invention to provide a composition and method useful in positioning a cementing composition in the annulus of an offshore well.
It is a further object of this invention to provide such a composition and method wherein the annulus can be completely filled with the cementing composition.
It is a still further object of this invention to provide such a composition and method wherein the filling of the annulus with cement is visually observable underwater.
It is another object of this invention to provide such a composition and method wherein the completeness of the displacement by the cementing composition of the fluid occupying the annulus following drilling and placement of the casing in the hole is improved.
Other objects, advantages and features of the invention will be apparent from the following description and claims.